For the most recent decade an obsessive discuss has raged about the disadvantages and advantages of programmable logic controllers (PLCs) versus PC-based control. Because of the different technology between PC and PLC diminish, with PLCs by using hardware of commercial off the shelf (COTS) and systems of PC incorporating real-time operating systems, a new technology of controllers, the PAC is rising. PAC, an acronym built by ARC (Automation Research Corporation), PAC is stand for Programmable Automation Controller and is utilized to explain a new industrial controller’s generation that join the functionality of a PC and a PLC. The acronym of PAC is being utilized both by traditional vendors of PLC to illustrate their high end systems and by PC control companies to explain their platforms of industrial control.
The Rule of 80-20
For the period of the three decades subsequent their introduction, PLCs have developed to communication over networks, integrate analog I/O, and new standards of programming for example IEC 61131-3. Nevertheless, the engineers make 80 percent of applications industrywith I/O digital, a few I/O analog points, and uncomplicated programming methods. The experts from ARC, VDC (Venture Development Corporation), and the online training source of PLC that is PLCS.net estimate that:
• 80% of application PLC tests are solved with a unit of 20 ladder logic instructions
• 72% of PLC digital I/O
• 77% of PLCs are utilized in small applications (fewer than 128 I/O)
Because 80% of applications industries are solved with conventional tools, there is high demand for uncomplicated low price PLCs. This has encouraged the development of low price micro PLCs with I/O digital that apply ladder logic. It has also produced a discontinuity in technology of controller, where 80 percent of applications need simple, low price controllers and 20 percent persistently drive the capabilities of conventional control systems. The applications that drop within the 20 percent are created by engineers who need higher loop rates, higher control algorithms, more capabilities of analog, and better combination with the network of enterprise.
In the years 80s and 90s, these 20 percenters assessed PCs for control of industry. The PC offered the software capabilities to carry out advanced jobs, provided a rich of graphical programming and user environment, and used components of COTS permitting control engineers to get benefit of technologies built up for other applications. These technologies comprise the processors of floating point; high speed busses of I/O, for example Ethernet and PCI; graphical development software tools; and non volatile data storage. The PC also offered unparalleled flexibility, advanced low-priced hardware, and greatly productive software.
On the other hand, PCs were still not perfect for control applications. Even though a lot of engineers utilized the PC when integrating advanced functionality, for example analog simulation and control, web based functionality, connectivity of database, and communication with 3rd party machines, the PLC still statute the control area The major problem with PC-based control was that PCs standard were not planned for rough environments.
The PC provided 3 major challenges:
1. Reliability: by non industrially hardened rotating and components magnetic hard drives, for example power supplies, the PC was better stability to malfunction.
2. Stability: frequently, the general-purpose of PC's operating system was not enough stable for control. The installations of PC-controlled were forced to handle crashes system and unintentional rebooting.
3. Unknown Programming Environment: the operators of plant require the capability to take priority over a system for maintenance and troubleshooting. They can force manually a coil to a preferred state, and rapidly patch the exaggerated code to speedily override a system using ladder logic. Nevertheless, the systems of PC need operators learn latest and higher tools.
Even though a number of engineers utilize special industrial computers with rough hardware and special operating systems, the majority engineers avoided using PCs for control because of PC reliability problems. As well, the devices utilized within a PC for diverse automation jobs, for example communications, I/O, or movement, may have dissimilar environments of development.
So the 20 percenters what's more lived with no functionality not simply skilled with a PLC or paved together a system that comprised a PLC for the portion of control the code and a PC for the more highly developed functionality. This is the reason a lot of factory floors at the moment have PLCs utilized in combination with PCs for the data logging, linking to scanners of bar code, putting in information into the databases, and bring out data to the Web. The huge problem with this setup type is that these systems are frequently not easy to build, maintain and troubleshoot. The system engineer frequently is left with the undesirable job of integrating software and hardware from multiple suppliers, which creates a challenge since the tools are not planned to operate together.